Most semiconductor integrated circuit processing sequences include steps which selectively or locally oxidize portions of the substrate. In a typical and pervasively used processing sequence, selected portions of the substrate are exposed by patterning a structure having, for example, oxide and silicon nitride layers, and then oxidizing the exposed portions of the substrate. The nitride layer is typically relatively thick as compared to the oxide layer to reduce oxide growth under the nitride layer. The oxidized substrate portions, termed field oxides, provide electrical isolation between individual devices.
Some devices are fabricated with processing sequences that use more than one selective local oxidation step. One such type device is termed an EEPROM which is an acronym for electrically erasable programmable read-only memory. The second oxidation is of a heavily doped region which is typically formed by ion implantation. The region is defined by depositing second oxide and nitride layers and patterning the nitride layer to expose selected portions of the oxide layer between the field oxides. The implanted region functions as the bit line, as well as the source/drain region, of a field effect transistor and the oxide formed by the second oxidation serves as a platform for the gate structure which controls the EEPROM device. For descriptions of exemplary structures see, for example, U.S. Pat. Nos. 4,750,024 and 4,853,895 issued on Jun. 7, 1988 and Aug. 1, 1991, respectively.
The ion implantation causes damage, i.e., creates defects, in the substrate which may adversely affect device operation. Furthermore, the second oxidation process may increase the defect density or cause the defects to propagate to otherwise good regions of the substrate. The nitride layer used for the first oxidation step can lift up and thus provide some stress relief. However, the nitride layer used for the second oxidation is on both the field oxide and the thin oxide region. The nitride layer is relatively thick as compared to the thin oxide layer. The nitride layer on the field oxide is effectively pinned in position as this region does not increase significantly in thickness during the second oxidation. This portion of the nitride layer can not lift up and provide stress relief, although the portion of the nitride layer between the field oxide regions can lift up and provide some stress relief.
Several approaches have been taken in attempts to reduce the defect density caused by ion implantation. See, for example, Journal of Vacuum Science and Technology, 16, pp. 342-344, March/April 1979 for the description of one approach. This paper reports a two-step annealing process which reduces the defect density caused by ion implantation. The method disclosed reduced the dislocation density by annealing in inert ambients for one hour at 550 degrees C. and then for one hour at 1000 degrees C.
Methods for reducing the dislocation density in process sequences which have two local oxidation steps are desirable.